It is known that a recovery boiler is used in the cellulose cooking process for recovering chemicals used in the cellulose cooking process and to produce steam for various phases in the process. After the cellulose cooking process, the spent cooking liquor, so-called black liquor, is separated from chemical pulp and led through an evaporation plant to be combusted in the recovery boiler. In the black liquor combustion process, the black liquor drops dry and the organic substance in it is burned. The remaining residue falls onto the floor of the furnace in the recovery boiler developing a so-called smelt layer, which has a conical shape. Combustion takes place in the smelt layer as well. From the smelt layer the smelt material is led to a dissolving tank. The wall tubes, as well as the floor tubes which constitute the floor of the recovery boiler, are most commonly made of fin tubes, which are connected together by welding to constitute planar constructions. Inside the tubes there is a cooling medium circulation, which is substantially a water-steam circulation.
The floor tubes constituting the floor of the recovery boiler are most commonly horizontally slightly inclined, so that a steam film would not develop inside the tube, but the cooling medium would flow steadily inside the tubes. The angle of inclination varies, the maximum angle being about 10.degree.. During the combustion process, the lowermost section of the smelt layer in the recovery boiler is solidified and thereby functions as an insulator protecting the floor tubes from excessively high temperatures.
The smelt on the floor of the recovery boiler is extremely corrosive. Furthermore, the high temperature of the smelt makes high demands on the material of the floor tubes. Commonly used as the floor tubes of a recovery boiler are compound tubes. A compound tube has a sandwiched wall structure, whereby the outer layer is of austenitic steel and the inner side is of carbon steel. Under normal process conditions, whereby the floor tubes are, on account of proper cooling, covered with solidified smelt material, the outer surface of the floor tube, i.e. the austenitic steel layer, is under compression stress. The reason for this is that austenitic steel has a higher thermal expansion coefficient than carbon steel. The temperature of the floor tubes is normally approximately equal to the temperature of the saturated vapor corresponding to the pressure of the recovery boiler, which on the other hand is dependent on the size and the design pressure of the recovery boiler. As an example could be mentioned a recovery boiler with a design pressure of 84 bar, whereby the temperature of the floor tubes is approximately 310.degree. C.
The floor is an extremely critical element to be designed for a recovery boiler. For one thing this is explained by the fact that if the cooling medium circulation in the floor tubes for some reason is disturbed, the result is that the temperature of the floor tubes rises very fast. Thus the steam developing inside the floor tube forms a layer on the upper part of the floor tube due to the disparity in density between steam and water. If the angle of inclination in relation to the horizontal plane is less than 9.degree. it is possible that the cooling medium circulation is disturbed by comparatively low heat fluxes, e.g. with a flow rate of 1 m/s even 20 kW/m.sup.2 causes disturbances.
It is also possible that under certain process conditions the smelt layer on top of the floor tubes constituting the floor becomes very thin or even that the floor of the recovery boiler becomes bare, whereby in this area a high heat flow is suddenly directed to the floor tubes, which causes a high temperature, which is even hundreds of grades higher than under normal process conditions.
When a thermal shock caused by the disturbances presented above,--either internal or external, or both,--is directed to the floor tubes, they fall under a high temperature. Under this condition it is very probable that the compression tension in the floor tube exceeds the yield strength of the floor tube material, i.e. the surface layer of the floor tube is upset. The force induced by compression no longer increases the tension but a permanent deformation results in the floor tube material. Consequently, when the temperature is restored to correspond to the normal process condition, e.g. when the cooling medium circulation is returned to normal, the cooling of the tube does not restore the tensions prevailing in the tube material, but a strong tensile stress prevails in the surface layer of the material and results in cracking of the floor tube.
Cracked floor tubes have to be replaced, because in case the cooling medium, i.e. water, flows into the furnace of the recovery boiler, a smelt explosion will result, damaging the boiler; in the worst case the entire boiler is destroyed and even human lives are lost. Repairs of the floor of a recovery boiler are difficult and expensive measures as the recovery boiler process has to be interrupted, i.e. the recovery boiler has to be run down and the floor has to be cleaned before it is possible to repair the floor tubes.
Furthermore, regarding the prior art, reference is made to WO-publication 92/18807. This publication suggests a power boiler whose vertical wall tube construction is composed of rifled tubes. However, the recovery boiler process differs substantially from the power boiler process. The first distinction relates to the fuel, which in the power boiler process is e.g. carbon. Black liquor is extremely corrosive, and the smelt layer accumulating on the floor of the recovery boiler in the recovery boiler process does not exist in any power boiler using fuel of a different type. On the other hand, power boilers have usually a grate on the bottom of the furnace, which is either a fluidizing grate, a travel grate or the like, depending on the type of the power boiler. Furthermore, the grate of a power boiler usually has openings for the removal of unburnt substance and coarse material whereas the floor of a recovery boiler constitutes an entirely closed construction.